Knurled nuts are used in low pressure applications and especially on plastic tubing where higher levels of torque are not required to make a sufficient seal for the application.
Tube fittings inserts reinforce the wall of flexible plastic tubing so that it can retain the shape and resistance to allow nuts and ferrules to seal on the tubing and fitting.
Stop collars prevent overtightening of tube fittings by mechanically restricting the advancement of the tube fitting nut at the maximum required to create a leak-tight seal.